I was just thinking......

We don't know what the materials are like that the make up the robots. We don't even know for sure that they are made from the same materials since the robots vary so wildly that they don't follow any of the rules we have for being a species here on Earth. Thus like cars on Earth perhaps some are more susceptible to a rust than others.

I'm also thinking that the weapon wasn't actually a rust weapon but that the side effect of the weapon doing it's job was to make it look like rust. The weapon could have shot some sort of nanites that disassemble at a microscopic level but as they consume, replicate and then fall off they look like rust.

Really? Not to be mean toeards any Ironhide fans, it did not seem slow on him.

Probably because said "rust" was being fired from an alien shapeshifter's giant-@$$ alien rust-esque weapon specifically designed to quickly eat away at things as opposed to "rust" in the sense of an old truck sitting in a farmer's field for four years as it gets rained on over and over again.

If only Sentinel had used the rust canon on Optimus when they were in the desert talking 'bout that Matrix.

I knew it! You want Sentinel to take over the Earth! You want him to reduce humanity to his slaves! What did he offer you to coerce this betrayal?! Power, money, women? It's all lies, Autovolt, all lies! He's using you, and when your usefulness is outlived, he'll make you a slave as well, forever condemned to rebuild a planet made of metal! ARGH!!